The present invention relates to integrated amplifiers, and, more particularly, to an audio amplifier and a technique of preventing irreversible damage to speakers and/or other components of an audio system.
An excessive offset on the outputs of amplifying channels coupled to the respective speakers may cause serious damage, and may even destroy the speakers. This problem is particularly acute in audio systems installed in vehicles, in which a breakdown of de-coupling capacitors installed between the audio processor and the amplifier or an infiltration of moisture, may often generate an excessive offset on the output terminals of the amplifying channels. Consequently, the offset is present on the speaker or speakers connected to them.
Manufacturers of these apparatus are increasingly demanding automatic detecting devices for detecting anomalous offset conditions on the outputs of the amplifying channels. A number of techniques have been proposed that are commonly based on the use of large external capacitors for integrating the output signals of an amplifying channel and extracting an average value therefrom.
Because of space saving requirements and limitations on the number of pins of a power package, integration of the output signals is performed in common on all amplifying channels. This arrangement may not be desirable if, for example, two channels simultaneously develop an anomalous offset of opposite signs.
There is a need and/or usefulness for a more effective method of detecting anomalous offset conditions which, besides precluding erroneous detections and having a high speed of detection, may be implemented with circuits that may be easily integrated with a reduced silicon area requirement.
The present invention provides a method and circuit effectively fulfilling the above noted requirements.
The method comprises establishing an interval or phase on the order of tens of milliseconds to measure the offset by applying to an input of an offset detection circuit a pulse of the same duration of the established interval with a certain frequency of repetition. The rising edge of the input pulse is detected by setting a bistable circuit whose output generates a logic signal signaling the existence of an offset outside a certain window. The method further includes resetting the bistable circuit upon the occurrence, after the initial set and for the entire duration of the detection period, of a signal amplitude within the window on the output nodes of the amplifying channel.
The detection period, i.e., the pulse duration, may have a value equal to the period of an audio band signal of a frequency generally not less than 10 Hz. The detection interval may have a fixed or variable duration established by the microprocessor that controls the audio system, generally between 50 and 100 milliseconds. The bistable circuit, which is commonly an SR latch, is set by a rising edge detector at the beginning of each detection phase of the offset. The bistable circuit is eventually reset during the detection phase if no anomalies are detected.
Once maximum negative and positive limit thresholds of the offset are fixed in an absence of an audio signal during a phase in which the channel is in a play mode, any offset that may be present on the output nodes of the amplifying channel is compared with the fixed thresholds (positive and negative). If the offset exceeds any of the fixed thresholds at any time during the detection period, an output logic signal indicating an abnormally large offset is generated. Every time the amplifier is turned on, with the audio amplifier muted and in absence of input signals, the system""s microprocessor may test the offset condition on the outputs of the various amplifying channels.
When the audio processor is in a play mode, there will be an audio signal fed to the input. The audio signal is summed to the offset on the output terminals of the amplifying channel. In this case, the detection of an anomalous offset condition would be required to hit a portion of the audio signal that has a sufficiently low amplitude such that during a whole detection interval, the sum of the signals on the output terminals (audio signal plus the offset) remains outside the window of tolerable values.
Indeed, the sum signal could remain inside the comparison notwithstanding an excessively large offset if opposite signs of the audio signal and the system would fail to detect the abnormal offset condition. However, it has been proven that for an interval of about 50-100 milliseconds, it is statistically extremely unlikely that a failure of the detection system would occur because the bandwidth of the audio signal is generally between 20 Hz and 20 KHz and, therefore, the audio signal crosses the zero level many times during a whole detection interval.
A particularly advantageous aspect of the method of the invention is its speed and effectiveness in detecting an anomalous condition as the offset increases towards dangerous levels. Moreover, it is possible with the method of the invention to command a continuous offset detection by simply imposing a frequency of repetition equivalent to the duration of the detection phase. This allows for a continuous check of the offset.
Naturally, both the detection period and the repeating frequency may be varied by the system""s microprocessor. The rupture or failure of a coupling capacitor is one of the most dangerous causes of a suddenly excessive offset because a DC signal is directly applied to the input of the audio amplifier. For example, consider a DC output voltage of an audio processor supplied at 8V=2*V(aP), and V(Csvr) is equal to the ground reference potential of the audio amplifier. For a coupling capacitor to rupture or fail, the input of the audio amplifier receives an applied voltage equal to V(Csvr)xe2x88x92V(aP) greater than  greater than 7xe2x88x924 greater than  greater than 3V.
Assuming 3V*26db=90V, the output will be completely clipped (Voffset greater than 12V). In such a case, the method of the invention after a first detection phase will signal the anomaly, thus indicating the existence of a rupture.
According to the present invention, an effective detecting circuit for detecting offset on the output nodes of an amplifying channel may include at least a bistable circuit having a reset input, a set input and an output coupled to a first input of a NOR gate. The NOR gate has a second input coupled to the input node for receiving a timing signal, and outputs a logic signal representative of the result of an offset detection phase.
The circuit further includes at least a rising edge detector having an input coupled to a source of the timing signal and an output coupled to the set input of the bistable circuit. A window comparator has inputs coupled to the output nodes of the amplifying channel, and an output coupled to a first input of a NAND gate. A second input of the NAND gate is coupled to the source of the timing signal, and an output is coupled to the reset input of the bistable circuit.